a. Related Application
Reference is hereby made to my copending application filed on the same date as the present application, Ser. No. 403,657, entitled "Spring Assembly" which is fully incorporated herein.
b. Field of the Invention
The present invention relates to variable ratio transmissions or drive systems generally utilizing a drive belt and, more particularly, to a mechanism for maintaining a substantially, constant, though minimized, tension on the drive belt. The invention has particular application to bicycles and is generally related in this context in the present disclosure. Nonetheless, such presentation must be understood as being exemplary of the invention and not limiting, since its principles may be applied in a host of other environments.
c. Description of the Prior Art
Bicycles equipped with chain-driven multi-speed transmissions called "derailleurs" have become popular world-wide. Derailleurs enable the bicycle rider to select various "speeds" for various slopes of the road and to accommodate the different needs of vigorous and weaker riders. A selected "speed" of a bicycle transmission identifies the ratio of the number of turns of the pedal crank to the resulting number of turns of the driven rear wheel.
A vigorous rider will choose a high speed or transmission ratio for riding down-hill or along a level road so that pedaling at a reasonable rate causes the bicycle to travel rapidly, and the same rider will choose a different ratio, a low speed, when riding slowly up-hill or on a poor road such as gravel.
Derailleurs with ten speeds are most common. However, derailleurs having more than ten speeds have become more and more common because they provide additional choices between the speeds of the ten speed transmission but have significant drawbacks including increased cost, weight and troublesome complexity.
A derailleur is inherently heavy due to its chain and many gears which are of metal, whereas bicycle riders have a high regard for lightweight bicycles. Also, derailleurs lack an indicator to tell the rider which speed or ratio has been selected. Furthermore, most derailleurs lack a housing so that the mechanism is exposed to rain, abrasive dirt, and other contaminants, leading to rust, wear and other damage. Finally, when a bicycle is equipped with a derailleur, it is difficult and cumbersome to remove and reassemble the rear wheel of the bicycle for replacing or repairing the tire. It is equally difficult to remove the whole transmission for replacement.
An alternative type of variable ratio transmission for bicycles is disclosed in U.S. Pat. Nos. 4,030,373 and 4,530,676 in which I am the inventor. Both of these patents are incorporated herein by reference. That type of transmission includes a pair of adjustable sheaves coupled to each other by a belt. The present invention involves a novel transmission of that type.
In U.S. Pat. No. 4,030,373, a set of sheave segments are continuously adjustable radially on each of the two sheaves, so that fine locking teeth are required to maintain any adjustment. In the transmission in U.S. Pat. No. 4,530,676, each of the sheaves also has a set of sheave segments that are adjustable. As an improvement over the '373 patent, the adjustments in '676 differ by discrete increments. A large number of discrete transmission ratios can be provided and relatively large locking teeth are used which are capable of withstanding much greater forces imposed on the sheave segments by the belt than in the '373 patent. Either of the sheaves in '676 can be used as the driving sheave in general applications of the transmission.
In any such transmission utilizing a belt, a continuous goal of the design is to maintain a substantially constant, albeit minimal, tension in the belt. Excessive tension occurring in the belt, even momentary as during shifting of the drive ratios, should be avoided. Various attempts have been made by me in seeking this goal. In the '373 patent, for example, I disclosed a construction in which the belt engages a pair of spaced idlers, one being fixed and the other being movable and biased against the belt. In the '676 patent, a somewhat different construction is disclosed in which a spaced pair of guide rollers are rotatably mounted on the respective ends of resilient arms. The guide rollers engage the drive belt and the resilient arms are free to pivot in the plane of the drive belt about an axis perpendicular to the plane of the drive belt.
A further transmission development of mine is presented in U.S. Pat. No. 4,816,008 and in its offspring, application Ser. No. 140,232, now U.S. Pat. No. 4,1816,008. Among the unique concepts disclosed in these disclosures is a belt tensioner according to which a pair of independently mounted and movable guide rollers are employed to engage the drive belt. These disclosures are also incorporated herein by reference.
Commonly assigned U.S. Pat. Nos. 4,820,244 and 4,832,660 disclose even further transmission developments, particularly relating to the construction of the sheaves which enable a variable ratio output, and are also incorporated herein by reference.
The present invention may be used in combination with the novel, improved transmission of the type mentioned above that includes two sheaves coupled to each other by a drive belt. In its broad aspect, the novel transmission is useful for widely varied purposes but certain attributes and added features render the transmission particularly useful in bicycles. In certain aspects of the invention, the novel transmission is an integral part of a bicycle.
The variable ratio drive mechanism has a drive sheave mechanism, a driven sheave mechanism, and an endless member or drive belt which drivingly couples the sheave mechanisms to each other. At least one of the sheave mechanisms includes a set of sheave segments, each sheave segment having a bearing surface engageable by the drive belt and means for placing and securing the sheave segments to said one sheave mechanisms in any of a series of positions distributed on said one sheave mechanism so that the bearing surfaces can be selectively positioned at a series of concentric circles. The drive belt applies pressure to all of the sheave segments except those in an adjustment zone of said one sheave mechanism where the drive belt is disengaged from the sheave segments. There is also provided gate means in the adjustment zone of said one sheave mechanism for adjusting the sheave segments to a selected position.
The gate adjustment means of the mechanism may have a control means and actuating means and the drive means to operate the mechanism may be in the same means to effect displacement of the gate to discrete positions. The mechanism may have two drive belt positioning means, one of which can be a force or tension applying means, adjacent the sheave mechanism for positioning the endless member to maintain wrap, and in some instances maximum wrap, of the endless member around the sheave mechanism, the drive belt positioning means being independently supported for movement. The mechanism may have locking rail means adjacent the sheave segments which are operated between released and locked conditions to adjust the positions of the sheave segments. The mechanism may also have a one-way clutch between the driven sheave means and the output thereof, such as the rear wheel of a bicycle, to provide easy decoupling of the output from the driven sheave means. The mechanism, which is easily and quickly removable from its frame such as a bicycle frame for replacement, intact, may also be made in a modular manner to be adaptable to frames of varying sizes.
It is eminently practical to provide a great many speeds, 21 speeds being provided in the example described in U.S. Pat. No. 4,816,008 in detail as compared to 10 speeds of a widely used derailleur. Molded plastic parts may be used almost exclusively so that the cost and weight of the transmission are comparatively low. The entire mechanism is designed such that it can readily be enclosed in a housing for protection against rain, dirt and other contaminants. This is in contrast to derailleurs wherein it is impractical to provide a housing and where exposure of the mechanism leads to rusting and premature wear of its parts.
The novel transmission can be equipped with an indicator to show the rider (or the user, in other applications of the transmission) which transmission ratio has been selected. In contrast, it is impractical to provide a unitary speed indicator in usual derailleurs.
In a bicycle equipped with the novel transmission with which the invention is utilized, the removal and reassembly of the rear wheel for repairing or replacing its tire is a simple matter. A one-way driving clutch between the ratio-changing mechanism and the rear wheel includes a driving clutch unit fixed to the belt driven sheave and a driven clutch unit fixed to the bicycle's rear wheel. The driven clutch unit and the rear wheel form an assembly that is removable from the bicycle as one part. When the rear wheel is removed, the driven clutch unit simple separates harmlessly from the driving clutch unit. The driving and driven units of the clutch become coupled to each other automatically when the rear wheel is reassembled to the bicycle. This is in contrast to a much more complicated procedure for removing the rear wheel of bicycles equipped with derailleurs.
In general applications of the transmission, either sheave can be used as the driving sheave. Moreover, the direction of rotation of the driving sheave can be reversed for special purposes when only a small amount of torque is required. When a bicycle is equipped with the novel transmission, its useful torque, when operated in reverse, enables the rider to change the transmission ratio by pedalling forward or backward, for example while coasting on a level road toward an uphill slope.
In transmissions having two belt-coupled sheaves, the belt includes a driving length that extends from one sheave to the other; the belt is wrapped part way around each sheave; and a return length of the belt completes the loop. Up to now, however, it has been difficult to assure that undesirable slack of the return length, or course, of the belt is taken up and that tension in the belt is maintained substantially constant throughout the shifting process.
It was in light of the foregoing that the present invention has been conceived and is now reduced to practice.